What a difficult thread. Any analogies along the lines of committing crimes against children by sexual crimes and this instance are initially very troubling. As well as juxpositioning it with females choosing not to carry a fetus. The only close compare is refusal to give a child a blood transfusion and it is not even a suitable one.
However, these types of medical interventions really need to be discussed whether or not it is refusal, or overly extended couses of treatment that are not humane.
When I exclude all of jas's bad analogies, from my deliberations, I find that I am basically on side with believing children should have a choice to, or not to, continue with inhumane medical treatment that may, or may not, make them better.
Having said that, the child in question here, has another medical condition that predisposes just allowing immediate personal choice. And I believe that there must be governmental oversight as to whether, or not, said child can make an truly informed choice. I have worked with FAS children who were more than competant to make such a decision, while others I have worked with would have not any understanding of implications, nor the ability to make such a choice for themselves.